An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum and is considered one of the basic elements for confinement and transmission of light over various distances. These devices are commonly used in a wide number of technical areas, including integrated optical circuits, long distance light wave communication, and biomedical imaging.
One type of optical waveguide is constructed as a solid body that is capable of transmitting light, such as a laser beam, therethrough while confining the light beam to a specific region of the solid body. A planar waveguide, also known as a slab waveguide, confines the light beam to a planar region within the waveguide, thereby guiding light in only one dimension.
Conventional solid state optical waveguides consist of a core, in which light is confined, and a cladding or substrate surrounding the core. A guided wave propagates in the waveguide along its longitudinal direction. For example, a conventional slab waveguide may include three layers of materials. When light is injected from the side into the middle layer, light may be confined in the middle layer by reflection. This occurs when the refractive index of the middle layer is larger than that of the surrounding layers. A waveguide in which the index profile has abrupt changes between the core and cladding is called a step-index waveguide, while one in which the index profile varies gradually is called a graded-index waveguide.
Optical communications propagating in a planar waveguide may be amplified when the waveguide is used in combination with a pumping laser. The pumping laser stimulates electronic transitions which amplify the communications signal as it passes through the waveguide. In order to retain this energy within the waveguide, one or more additional layers of material may be added to the waveguide. This material should possess several characteristics, including the ability to absorb and reflect light back into the core and/or cladding. In addition, the material should possess good bonding and robust mechanical properties that allow it to interface with other materials and/or layers that construct the waveguide.